A wind power plant comprises a number of wind turbine generators (WTG), each comprising a turbine connected to a generator for converting mechanical energy in the turbine to electrical energy. The wind power plant is further connected to an electrical power network for providing the electrical energy generated by the WTGs to the power network.
The power network should have defined parameters, in particular a defined voltage and a defined frequency, e.g. 50 Hz. The stability of the power network parameters is dependent on a variety of variables including the balance between generated power and consumed power in the network at every instant. Any imbalance leads to changes in the network frequency. When more power is generated than is consumed in the network, the frequency increases. When more power is consumed than generated, the frequency decreases. It is important to have a stable frequency in the network, i.e. to keep the frequency fluctuations as small as possible.
In a wind-driven power plant, each wind turbine is preferably driven at a variable speed, which speed is adapted to the wind speed, for performance reasons and because otherwise there is an increase in the stress suffered by the mechanical parts of the turbine. Since the turbines are driven at variable speed, their individual rotational speed is decoupled from the frequency of the power network. Consequently, there is some freedom between the frequency of the power network and the rotational speed of the turbines, which means that the power delivered by a wind power plant is independent of the current frequency of the power network.
Since, generally, the number of wind power plants in a power network increases, there is an increasing demand from network operators to be able to control the power delivered by the wind power plants according to frequency variations in the network.
EP1467463 and US2007/0085343 deals with controlling the real power output of a wind power plant connected to a power network to contribute to power network stabilization by controlling the real power output of the wind power plant depending on the value of the network frequency. I.e. when the frequency is high in the network, the wind power plant is controlled to decrease the real power provided to the network, and vice versa. US 2007/0085343 especially discusses using the inertia of the wind turbines for, at low network frequencies, being able to deliver more power than nominal power to the network for a short period. Although, such a frequency stabilisation could only be useable for a short while. Thereby, it is important to control the use of the inertia such that it is not used too long such that the rotational speed of the turbines will become too low or even stop.
There is a need for alternative ways of controlling the real power output of a wind power plant to be able to contribute to power network frequency stabilization.